The number and variety of portable and mobile devices in use have exploded in the last decade. For example, the use of mobile phones, tablets, media players etc. has become ubiquitous. Such devices are generally powered by internal batteries and the typical use scenario often requires recharging of batteries or direct wired powering of the device from an external power supply.
Most present day systems require a wiring and/or explicit electrical contacts to be powered from an external power supply. However, this tends to be impractical and requires the user to physically insert connectors or otherwise establish a physical electrical contact. It also tends to be inconvenient to the user by introducing lengths of wire. Typically, power requirements also differ significantly, and currently most devices are provided with their own dedicated power supply resulting in a typical user having a large number of different power supplies with each being dedicated to a specific device. Although, the use of internal batteries may avoid the need for a wired connection to a power supply during use, this only provides a partial solution as the batteries will need recharging (or replacing which is expensive). The use of batteries may also add substantially to the weight and potentially cost and size of the devices.
In order to provide a significantly improved user experience, it has been proposed to use a wireless power supply wherein power is inductively transferred from a transmitter coil in a power transmitter device to a receiver coil in the individual devices.
Power transmission via magnetic induction is a well-known concept, mostly applied in transformers, having a tight coupling between primary transmitter coil and a secondary receiver coil. By separating the primary transmitter coil and the secondary receiver coil between two devices, wireless power transfer between these becomes possible based on the principle of a loosely coupled transformer.
Such an arrangement allows a wireless power transfer to the device without requiring any wires or physical electrical connections to be made. Indeed, it may simply allow a device to be placed adjacent to or on top of the transmitter coil in order to be recharged or powered externally. For example, power transmitter devices may be arranged with a horizontal surface on which a device can simply be placed in order to be powered.
Furthermore, such wireless power transfer arrangements may advantageously be designed such that the power transmitter device can be used with a range of power receiver devices. In particular, a wireless power transfer standard known as the Qi standard has been defined and is currently being developed further. This standard allows power transmitter devices that meet the Qi standard to be used with power receiver devices that also meet the Qi standard without these having to be from the same manufacturer or having to be dedicated to each other. The Qi standard further includes some functionality for allowing the operation to be adapted to the specific power receiver device (e.g. dependent on the specific power drain).
The Qi standard is developed by the Wireless power Consortium and more information can e.g. be found on their website: http://www.wirelesspowerconsortium.com/index.html, where in particular the defined Standards documents can be found.
The Qi wireless power standard describes that a power transmitter must be able to provide a guaranteed power to the power receiver. The specific power level needed depends on the design of the power receiver. In order to specify the guaranteed power, a set of test power receivers and load conditions are defined which describe the guaranteed power level for each of the conditions.
Qi originally defined a wireless power transfer for low power devices considered to be devices having a power drain of less than 5 W. Systems that fall within the scope of this standard use inductive coupling between two planar coils to transfer power from the power transmitter to the power receiver. The distance between the two coils is typically 5 mm. It is possible to extend that range to at least 40 mm.
However, work is ongoing to increase the available power, and in particular the standard is being extended to mid-power devices being devices having a power drain of more than 5 W.
The Qi standard defines a variety of technical requirements, parameters and operating procedures that a compatible device must meet.
Communication
The Qi standard supports communication from the power receiver to the power transmitter thereby enabling the power receiver to provide information that may allow the power transmitter to adapt to the specific power receiver. In the current standard, a unidirectional communication link from the power receiver to the power transmitter has been defined and the approach is based on a philosophy of the power receiver being the controlling element. To prepare and control the power transfer between the power transmitter and the power receiver, the power receiver specifically communicates information to the power transmitter.
The unidirectional communication is achieved by the power receiver performing load modulation wherein a loading applied to the secondary receiver coil by the power receiver is varied to provide a modulation of the power signal. The resulting changes in the electrical characteristics (e.g. variations in the current draw) can be detected and decoded (demodulated) by the power transmitter.
Thus, at the physical layer, the communication channel from power receiver to the power transmitter uses the power signal as a data carrier. The power receiver modulates a load which is detected by a change in the amplitude and/or phase of the transmitter coil current or voltage. The data is formatted in bytes and packets.
More information can be found in chapter 6 of part 1 the Qi wireless power specification (version 1.0).
Although Qi uses a unidirectional communication link, it has been proposed to introduce communication from the power transmitter to the power receiver. However, such a bidirectional link is not trivial to include and is subject to a large number of difficulties and challenges. For example, the resulting system still needs to be backwards compatible and e.g. power transmitters and receivers that are not capable of bidirectional communication still need to be supported. Furthermore, the technical restrictions in terms of e.g. modulation options, power variations, transmission options etc. are very restrictive as they need to fit in with the existing parameters. It is also important that cost and complexity is kept low, and e.g. it is desirable that the requirement for additional hardware is minimized, that detection is easy and reliable, etc. It is also important that the communication from the power transmitter to the power receiver does not impact, degrade or interfere with the communication from the power receiver to the power transmitter. Furthermore, an all-important requirement is that the communication link does not unacceptably degrade the power transfer ability of the system.
Accordingly, many challenges and difficulties are associated with enhancing a power transfer system such as Qi to include bidirectional communication.
System Control
In order to control the wireless power transfer system, the Qi standard specifies a number of phases or modes that the system may be in at different times of the operation. More details can be found in chapter 5 of part 1 the Qi wireless power specification (version 1.0).
The system may be in the following phases:
Selection Phase
This phase is the typical phase when the system is not used, i.e. when there is no coupling between a power transmitter and a power receiver (i.e. no power receiver is positioned close to the power transmitter).
In the selection phase, the power transmitter may be in a stand-by mode but will sense in order to detect a possible presence of an object. Similarly, the receiver will wait for the presence of a power signal.
Ping Phase:
If the transmitter detects the possible presence of an object, e.g. due to a capacitance change, the system proceeds to the ping phase in which the power transmitter (at least intermittently) provides a power signal. This power signal is detected by the power receiver which proceeds to send an initial package to the power transmitter. Specifically, if a power receiver is present on the interface of the power transmitter, the power receiver communicates an initial signal strength packet to the power transmitter. The signal strength packet provides an indication of the degree of coupling between the power transmitter coil and the power receiver coil. The signal strength packet is detected by the power transmitter.
Identification & Configuration Phase:
The power transmitter and power receiver then proceeds to the identification and configuration phase wherein the power receiver communicates at least an identifier and a required power. The information is communicated in multiple data packets by load modulation. The power transmitter maintains a constant power signal during the identification and configuration phase in order to allow the load modulation to be detected. Specifically, the power transmitter provides a power signal with constant amplitude, frequency and phase for this purpose (except from the change caused by load-modulation).
In preparation of the actual power transfer, the power receiver can apply the received signal to power up its electronics but it keeps its output load disconnected. The power receiver communicates packets to the power transmitter. These packets include mandatory messages, such as the identification and configuration packet, or may include some defined optional messages, such as an extended identification packet or power hold-off packet.
The power transmitter proceeds to configure the power signal in accordance with the information received from the power receiver.
Power Transfer Phase:
The system then proceeds to the power transfer phase in which the power transmitter provides the required power signal and the power receiver connects the output load to supply it with the received power.
During this phase, the power receiver monitors the output load conditions, and specifically it measures the control error between the actual value and the desired value of a certain operating point. It communicates these control errors in control error messages to the power transmitter with a minimum rate of e.g. every 250 msec. This provides an indication of the continued presence of the power receiver to the power transmitter. In addition the control error messages are used to implement a closed loop power control where the power transmitter adapts the power signal to minimize the reported error. Specifically, if the actual value of the operating point equals the desired value, the power receiver communicates a control error with a value of zero resulting in no change in the power signal. In case the power receiver communicates a control error different from zero, the power transmitter will adjust the power signal accordingly.
A potential problem with wireless power transfer is that power may unintentionally be transferred to e.g. metallic objects. For example, if a foreign object, such as e.g. a coin, key, ring etc., is placed upon the power transmitter platform arranged to receive a power receiver, the magnetic flux generated by the transmitter coil will introduce eddy currents in the metal objects which will cause the objects to heat up. The heat increase may be very significant and may indeed result in a risk of pain and damage to humans subsequently picking up the objects.
Experiments have shown that metal objects positioned at the surface of a power transmitter can reach an undesired high temperature (higher than 60° C.) at normal environment temperatures (20° C.) even for power dissipation in the object being as low as 500 mW. For comparison, skin burning caused by contact with hot objects starts at temperatures of around 65° C. The experiments have indicated that a power absorption of 500 mW or more in a typical foreign object rises its temperature to an unacceptable level.
In order to prevent such scenarios, it has been proposed to introduce foreign object detection where the power transmitter can detect the presence of a foreign object and reduce the transmit power. For example, the Qi system includes functionality for detecting a foreign object, and for reducing power if a foreign object is detected.
The power dissipation in a foreign object can be estimated from the difference between transmitted and received power. In order to prevent that too much power is dissipated in a foreign object, the transmitter can terminate the power transfer if the power loss exceeds a threshold.
In the current Qi Standard the preferred approach is to determine the power loss across the interface between the power transmitter and the power receiver in order to determine any loss in foreign objects. For this purpose, the power receiver estimates the amount of power that enters its interface surface—i.e. the received power. In order to generate the estimate, the power receiver measures the amount of power provided to the load, and adds an estimate of the losses in components—coil, resonant capacitor, rectifier, etc., as well as losses in conductive elements of the device, such as in metal parts that are not exposed to the user. The power receiver communicates the determined received power estimate to the power transmitter at regular intervals.
The power transmitter estimates the amount of power extracted from the power signal—i.e. the transmitted power. The power transmitter can then calculate the difference between the transmitted power and the received power, and if the difference exceeds a given level, the power transmitter may determine that a situation has occurred where an unacceptable power may be dissipated in a foreign object. For example, a foreign object may be positioned on or near the power transmitter resulting in this being heated due to the power signal. If the power loss exceeds a give threshold, the power transmitter terminates the power transfer in order to prevent the object from getting too hot. More details can be found in the Qi Standard, System Description Wireless power.
When performing this power loss detection, it is important that the power loss is determined with sufficient accuracy to ensure that the presence of a foreign object is detected. Firstly, it must be ensured that a foreign object which absorbs significant power from the magnetic field is detected. In order to ensure this, any error in estimating the power loss calculated from the transmitted and received power must be less than the acceptable level for power absorption in a foreign object. Similarly, in order to avoid false detections, the accuracy of the power loss calculation must be sufficiently accurate to not result in estimated power loss values that are too high when no foreign object is present.
It is substantially more difficult to determine the transmitted and received power estimates sufficiently accurately at higher power levels than for lower power levels. For example, assuming that an uncertainty of the estimates of the transmitted and received power is ±3%, this can lead to an error of                ±150 mW at 5 W transmitted and received power, and        ±1.5 W at 50 W transmitted and received power.        
Thus, whereas such accuracy may be acceptable for a low power transfer operation it is not acceptable for a high power transfer operation.
Typically, it is required that the power transmitter must be able to detect power consumption of foreign objects of only 350 mW or even lower. This requires very accurate estimation of the received power and the transmitted power. This is particularly difficult at high power levels, and frequently it is difficult for power receivers to generate estimates that are sufficiently accurate. However, if the power receiver overestimates the received power, this can result in power consumption by foreign objects not being detected. Conversely, if the power receiver underestimates the received power, this may lead to false detections where the power transmitter terminates the power transfer despite no foreign objects being present.
In order to obtain the desired accuracy, it has been proposed that the power transmitter and power receiver are calibrated to each other before power transfer at least at higher levels is performed. However, although such an approach may be desirable in many scenarios, it may also be considered inconvenient to the user as such calibrations may at best delay the power transfer, and may in many scenarios require user involvement before power transfer can proceed.
An improved power transfer system would be advantageous. In particular, an approach that allows improved operation while maintaining a user friendly approach would be advantageous. Particularly, an approach that allows easier user operation while ensuring safe operation, especially at higher power levels, would be advantageous. An improved power transfer system allowing increased flexibility, facilitated implementation, facilitated operation, safer operation, reduced risk of foreign object heating, increased accuracy and/or improved performance would be advantageous.